A semiconductor single crystal is a material composed of a periodic and regular configuration of specific atoms. Its electron propagation property is determined by the interatomic spacing in the semiconductor crystal. Namely, a semiconductor has an energy band gap, and the energy band gap is determined depending on the wave nature of electrons and the periodic potential of atoms.
On the other hand, a photonic crystal is a material having optical potential differences, that is, a three-dimensional structure composed of substances with refractive index differences arranged in periods equivalent to light wavelengths. Such a photonic crystal was suggested by Yablonovich and others.
In a photonic crystal, the optical propagation property is limited by a binding condition concerning the wave nature of light. Namely, optical propagation in a photonic crystal is limited as well as electron propagation in a semiconductor. In a photonic crystal, an optical forbidden band, that is, a photonic band gap exists, and due to this existence, light in a specific waveband cannot propagate in the crystal.
Conventionally, various photonic crystals have been suggested. For example, a photonic crystal composed of submicron particles arranged in periods equivalent to light wavelengths exists. For microwaves, a photonic crystal composed of polymer particles arranged in a space is generally known.
In addition to these, there are various photonic crystals such as a photonic crystal that has periodic minute spaces formed in a metal by chemically dissolving polymer particles after solidifying the polymer particles in the metal, a photonic crystal with pores perforating at equal intervals into a metal, a photonic crystal which includes regions that are formed in a solid material by a laser so as to be different in refractive index from other regions, and a photonic crystal of photo-induced polymers formed in a groove shape by means of lithography. A photonic crystal formed by the abovementioned processing has a photonic band gap uniquely determined by the structure.
A wavelength tunable light source unit using such a photonic crystal can select and output a predetermined wavelength range of input light. In the description given below, light to be inputted into the photonic crystal is regarded as input light, and light to be outputted from the photonic crystal after being transmitted through the photonic crystal is regarded as output light.